It is known to use pollutant-detecting gas sensors for the control of fresh air/ambient air valves. These sensors additionally may be employed for switching on- and off filter devices in motor vehicle cabins and protection cabins for equipment operators. Furthermore, it is known that these semiconductor gas sensors, due to their measuring mode and manufacture, have tolerances too high for this type of application.
A great number of proposals have been made to improve the sensitivity and stability of these sensors. These have, however, not been successful. For instance, when the pollutant-detecting gas sensors are used in motor vehicles, the transverse sensitivity towards various travel speeds, temperatures, degrees of humidity and so on, is still unacceptably high.
In order to eliminate these disadvantages, the invention proposes that a gas sensor should be exposed alternately to the gas streams to be evaluated. The sensor is mounted at the separation point between the outside air and the cabin air. Thereby through switching, effected within seconds, practically a constant comparison between the quality of the inner air and the outer air is established. For this purpose, the gas sensor cooperates with a switching device. The gas sensor and the switching device can be arranged on a fresh air/ambient air valve. The output signals of the gas sensor serve for controlling the fresh air/ambient air valve.
It is known that semiconductor sensors, particularly of the stannic dioxide variety, have a very high transverse sensitivity towards temperature- and humidity variations such as in the degree of humidity. Sometimes the variations are so high that they can lead to deviations up to 70% in the measured value, namely due to various humidity degrees. The influence of temperature variations can have similar effects. It is known to create a temperature compensation by means of temperature sensors. Humidity can be technically measured only as relative humidity. This measured value, however, leads to very different absolute humidities at various air temperatures. Measurement of the realtive humidity thus cannot be directly used for the correction of the humidity flow. As a result, the correction becomes very complicated and can be generally figured out only by a computer.